So, under services->manage services, there's a new 'inc/dec rating' (for increment/decrement). It works pretty much like all the other ratings, and goes in the same places, but it is a simple number counter--left-click to add, right to subtract. Middle-click lets you edit directly. I've wired it up for system:rating, too, with only one oddness--since every file starts at count 0, there's no concept of 'not rated' for this service.